1. Field of the Invention
This invention relates generally to a projector apparatus and more particularly is directed to a color projector apparatus of single cathode ray tube type by which a color picture such as a television video picture or the like is magnified and then projected onto a screen.
2. Description of the Prior Art
In the art, various projector apparatuses of this kind have been proposed. As, for example, shown in FIG. 1, a projector apparatus is proposed which includes three cathode ray tubes, 1R, 1G and 1B which reproduce picture images of three primary color components of red, green and blue, respectively. The reproduced red, green and blue picture images from these cathode ray tubes 1R, 1G and 1B are respectively magnified and projected onto a common screen 3 by corresponding lens systems, 2R, 2G and 2B in such a way that the reproduced picture images from the respective cathode ray tubes, 1R, 1G and 1B are synthesized one another on the common screen 3 as a color image. In this case, the respective picture of each color is formed by each monochromatic cathode ray tube and hence the bright synthesized picture image can be projected. Since the projected images of respective colors have to be coincident with one another with precision on the screen 3, the position of the respective lens systems, 2R, 2G and 2B relative to the cathode ray tubes, 1R, 1G and 1B are fixed and a distance between the projection screen 3 and each of the lens systems, 2R, 2G and 2B is fixed, giving rise to a restriction in the use thereof. Moreover, color shading occurs depending on the position of a viewer.
Whereas, as shown in FIG. 2, there has been proposed another projector apparatus in which red, green and blue picture images from monochromatic cathode ray tubes, 1R, 1G and 1B are respectively passed through two dichroic mirrors, 4a and 4b, coincided one another on the optical axis of a common lens 2 and magnified thereby and then projected onto a screen 3 as a synthesized color picture image. In this case, the distances among the cathode ray tubes, 1R, 1G and 1B of respective colors, namely, the object points and the common lens system 2 are respectively determined substantially equal. This distance, however, can not be reduced sufficiently due to the existence of the dichroic mirrors, 4a and 4b, requiring the use of the lens system 2 of long focal length. Thus, if the lens system 2 is formed of a bright lens (that is, of which the F-number is lower), a lens of larger aperture is required.
As other example of prior art projector apparatus, there has been proposed such an arrangement as illustrated in FIG. 3 in which two dichroic mirrors, 4a' and 4b' are intersected each other and placed at the same position. In this case, although the distance between each of the cathode ray tubes, 1R, 1G and 1B and the lens system 2 can be selected relatively small, the assembling and manufacturing processes become significantly troublesome in practice. Moreover, there is a defect that the color is deteriorated by the reflection of the light on the two dichroic mirrors, 4a' and 4b'.
For further example, as illustrated in FIG. 4, there has been proposed such a projector apparatus, which comprises one monochromatic cathode ray tube, for example, cathode ray tube 1G to produce a color component picture image of green and a cathode ray tube 1RB to produce two other color components of picture images, red and blue. The reproduced picture images therefrom are synthesized by a single dichroic mirror 4 and then projected onto a screen 3 through a common lens system 2 as a color picture image. In this case, the two component picture images of red and blue are provided by the common cathode ray tube 1RB and the green picture image of high visual sensitivity is provided by the monochromatic cathode ray tube 1G as a bright picture image thus the relatively bright color projected picture image being made. As compared with the prior art projector apparatuses of 3-tube type in FIGS. 1 to 3, the projector apparatus of the last example has such an advantage that this apparatus can be made compact in size and the assembling and manufacturing processes thereof become simple and easy.
Now, let us consider the cathode ray tube used in the projector apparatus. In the cathode ray tube, in order to reproduce a picture image of high brightness, the energy of electron beam to excite the phosphor screen thereof has to be increased, namely, an acceleration voltage of electron beam has to be increased or its beam current has to be increased. While, in the cathode ray tube of which the phosphor screen is separately coated with two kinds of light-emitting phosphor substances or more to provide picture images of more than two primary colors, for example, in the cathode ray tube 1RB of two picture images used in the prior art projector apparatus as shown in FIG. 4, the continuous use of this type cathode ray tube causes a color shift and the deterioration of color hue and at last, no bright color picture image can not be produced. The reason for this is that, the ordinary color cathode ray tube includes behind its phosphor screen an electron beam landing position determining electrode such as a shadow mask or an aperture grille provided with apertures through which the electron beam for respective colors lands on the respective color phosphor substances to determine the landing position of the electron beam on the phosphor screen. Accordingly, if the energy of electron beam is increased or the beam current thereof is increased in order to reproduce a bright picture not only the temperature of the phosphor screen is raised due to the impingement of electron beam thereon, but also the temperature of the electron beam landing position determining electrode is raised considerably because about 80 percent of the energy of electron beam is converted into heat by the impingement of electron beam on this electrode. Due to the fact that the inside of the tube envelope is kept at high vacuum degree, the heat generated on this electrode is difficult to be transferred to the outside therefrom so that the temperature of the electrode is raised to a rather high temperature. This leads to a great thermal expansion of the electrode such as the shadow mask or the aperture grille and then slackening occurs in these electrodes and cause the electron beam mislanding on the wrong position which produces color shift. While, due to the rise of temperature on the shadow mask or the aperture grille and further the impingement of electron beam on the phosphor screen itself, the significant rise of temperature on the phosphor screen is caused. The rise of temperature on the phosphor screen causes the deterioration in its illuminated brightness, namely, so-called thermal quenching of phosphor occurs. This thermal quenching is such a phenomenon that the brightness of phosphor substance is lowered with the rise of temperature thereof. Since the degree of the thermal quenching is different depending on the phosphor substances of respective colors, the white balance will become out of order. The disorder of the white balance is particularly remarkable in the center of the phosphor screen because the heat radiation effect is poor particularly in the center of the phosphor screen and the rise of temperature thereat is significant, leading to a disastrous deterioration of picture quality.
There has been proposed a so-called beam index type-color cathode ray tube to avoid the provision of the electron beam landing position determining electrode such as the shadow mask or the aperture grille facing to the phosphor screen used in the prior art cathode ray tube which can produce picture images of more than two colors as, for example, shown in FIG. 4. As is known well, such previously proposed beam index-type color cathode ray tube has a configuration such that on a phosphor screen formed of plural color phosphors to be a color phosphor pattern is located a detection section with a predetermined positional relation to the phosphor pattern to detect the scanning position of electron beam and the electron beam is controlled by the index signal to scan the predetermined phosphor. Consequently, unlike the cathode ray tube of ordinary shadow mask type or aperture grille type in which the electron beam landing position determing electrode such as the shadow mask or aperture grille is opposed to the phosphor screen formed on the inner surface of the panel, the cathode ray tube of beam index type can avoid the loss of the beam power caused by the masking with the electron beam landing position determining electrode thus providing the picture image brighter than that of the cathode ray tube of shadow mask type or aperture grille type.
However, recently, it has been requested or desired that the color projector apparatus has a more compactness, a simple construction, simplified assembling and manufacturing and easy handling. In addition, the demand of bright picture image of the projector apparatus is being increased. In this connection, even the color projector apparatus of 2-tube type explained in connection with FIG. 4 can not fully satisfy the requirements of the compactness and still has a problem that it is quite troublesome to determine the positional relationship of the two cathode ray tubes. Furthermore, even when as the cathode ray tube of 2-tube type color projector apparatus in FIG. 4, the cathode ray tube 1RB of beam index type is employed, due to the thermal quenching by the impingement of electron beam on the phosphor screen, the sufficient improvement of brightness can not be performed.